1. Field of the Invention
The present invention relates to high density memory devices based on phase change memory materials and particularly sensing circuitry for such devices.
2. Description of Related Art
Phase change memory materials are widely used in read-write optical disks. These materials have at least two solid phases, including for example a generally amorphous solid phase and a generally crystalline solid phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.
Phase change memory materials, like chalcogenide based materials and similar materials, also can be caused to change phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous phase is characterized by higher resistivity than the generally crystalline phase, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.
Modulating between amorphous and crystalline phases is achieved by controlling the thermal energy to which the phase change material is subjected. For example, modulating the phase change material from the amorphous phase to the crystalline phase is achieved by heating the same to a temperature between the glass transition temperature of the phase change material and the melting temperature. This is referred to as “set” and occurs during a relatively low current operation. Modulating from the crystalline phase to the amorphous phase, referred to as “reset,” occurs during a relatively high current operation during which melting of the phase change material occurs followed by rapidly cooling the same below its glass transition temperature at a rate to reduce, if not prevent, nucleation and growth of crystallites. To that end, the phase change material may be subjected to a short high current density pulse to melt or break down the crystalline structure so that at least a portion of the phase change structure stabilizes in the amorphous phase at ambient temperatures. By controlling the amount of crystalline and amorphous phases of the material in a phase change element, it is possible to establish multiple memory states in the element, including a reset state comprising a substantially all-amorphous phase in an active region of the material, one or more intermediate states in which mixtures of amorphous phase and crystalline phase in the active region of the material are formed, and a set state comprising a substantially all-crystalline phase in the active region of the material.
During a read operation the phase change material is subjected to a read pulse in order to determine the resistance of the memory element indicating whether the phase change material is in a set state, a reset state or in an intermediate state. It is desirable, however, to select a suitable read pulse so that the relative amounts of amorphous and crystalline phases of the phase change material are not disturbed during the read operation.